Improving semi-arid agroecosystem services with cover crop mixes

Winter wheat (Triticum aestivum, L.) production in the semi-arid US Northern High Plains (NHP) is challenged by frequent droughts and water-limited, low fertility soils. Composted cattle manure (compost) and cover crops (CC) are known to provide agroecosystem services such as improved soil health, and in the CC case, increased plant diversity, and competition with weedy species. The main concern of planting CC in winter wheat fallow rotation in regions that are more productive than the NHP, however, is the soil moisture depletion. It is unknown however, whether addition of CC to compost-amended soils in the NHP will improve soil properties and agroecosystem health without compromising already low soil water content. The main objective of this study was to assess the effects of four CC treatments amended with compost (45 Mg ha-1) or inorganic fertilizer (IF) (.09 Mg ha-1 mono-ammonium phosphate, 11-52-0 and 1.2 Mg ha-1ammonium sulfate, 21-0-0) on the presence of weeds, soil and plant total carbon (C), nitrogen (N), and biological dinitrogen (N2) fixation (BNF). Mycorrhizal Mix (MM), Nitrogen Fixer Mix (NF), Soil Building Mix (SB), a monoculture of phacelia (Phacelia tanacetifolia Benth L.) (PH), and a no CC control (no CC) were grown in native soil kept at 7% soil moisture in a greenhouse for a period of nine weeks. When amended with compost, MM was the most beneficial (48 g m-2 BNF and 1.7% soil C increase). SB had the highest germination, aboveground biomass, and decreased weed biomass by 60%. It also demonstrated the second highest amount of BNF (40 g m-2) and soil C increase by 1.5%. On contrary, IF hindered BNF by almost 70% in all legume-containing CC treatments and reduced soil C by 15%.


Introduction
The inclusion of cover crops (CC) in crop rotations have been successfully practiced worldwide, yet the CC adoption in cold, semi-arid environments, such as the US Northern High Plains (NHP), is very limited.When planted during the "off-crop season," CC can help build up soil organic matter (SOM), improve soil aggregation, prevent erosion, and reduce nutrient loss [1].In addition to soil benefits, CC also provide effective weed biomass reduction [2].There are concerns however, that CC draws down soil moisture and compromise winter wheat (Triticum aestivum, L.) yield [3].
The NHP has a growing area under organic winter wheat production and active soil health and weed management strategies are needed to help producers maintain their certification.The most common CC mix recommended locally is field pea (Pisum sativum L.) mixed with oat (Avena sativa L.).Exploring alternative CC mixes that help fulfill production goals and aid in management objectives are, however, much needed as this mix often experiences very poor establishment.
The term "cover crops" refers to monocultures or mixes of two or more plant species with various traits.Species from plant families such as Poaceae, Fabaceae, Brassicaceae and Asteraceae are commonly used in CC mixes.Each family has beneficial characteristics for agroecosystems.Fibrous roots of plants from the Poaceae family are known to stabilize soil, capture nutrients, and produce high biomass that returns organic matter (OM) to the ground upon termination [4].Species from the Fabaceae family live in symbiosis with bacteria capable of atmospheric dinitrogen (N 2 ) fixation.This relationship supplies nitrogen (N) to the plants during the growing season.Upon plant senescence, N in plant biomass is deposited in the soil [5].Species from the Brassicaceae family put out deep taproots that help alleviate soil compaction.Seeds are fast to germinate, and hence, have a time advantage to establish themselves ahead of weeds [6].Sunflower (Helianthus annuus) from the Asteraceae family is used in CC mixes to improve soil hydraulic properties.Sunflower roots access water deeper in soil layers and redistribute it near the soil surface to benefit shallow rooted species in a CC mix [7].
Comparing 15 N natural abundance (δ 15 N) between a soil, plant, inorganic fertilizer or compost provides many insights on the pathways of plant N uptake and soil organic matter decomposition (He et al., 2009).This approach is based on the premise that the majority of naturally occurring N is in the 14 N form (99.6%) and the remaining 0.4% of N exists as 15 N [8].The ratio of these two isotopes ( 15 N/ 14 N) in the atmosphere is constant at .0036765 and is used as a reference.Many microbial and plant physiological processes discriminate against heavier 15 N compared to 14 N and can indicate what forms of N are available to the plant [9].For example, during the composting process, microbes use the 14 N resulting in greater compost 15 N enrichment.When inorganic fertilizer is manufactured from atmospheric N, the 15 N level is of the fertilizer is the same as the atmosphere, which is close to zero.As cover crops and weeds compete for the N provided from these inputs, plant tissue 15 N concentrations will increase or decrease accordingly.These two inputs with very different 15 N concentrations provide a traceable N source and observations on cover crop versus weedy species competition can be made.
During atmospheric N 2 fixation by bacteria living on leguminous roots, the soil bacteria discriminate against 15 N as they only utilize the lighter isotope 14 N.This discrimination ratio is transferred to the legume tissue and provides a traceable N source when observing legume plant biological N fixation BNF [8,9].
Soil amendments, like composted cattle manure (compost) and inorganic fertilizer (IF) are used to boost cash crop performance and yield [10,11] but how these soil amendments impact CC performance has not been researched.The main objective of this project was to observe how CC mixtures and weedy species interact with, respond to, and compete for soil N sourced from soil amendments under low water conditions.

Site description
This study was carried out for a period of nine weeks in the spring of 2019 at the University of Wyoming Laramie Research and Education Center (LREC) Greenhouse in Laramie, Wyoming (41.31˚N, 105.59˚, 2214 meters above sea level).This length of time was chosen arbitrarily to mimic the short window of time, during which cover crops are planted, produce biomass, flower and are either harvested or terminated, depending on the production goals.The greenhouse facility was not equipped with any artificial light and the average light intensity averaged 23,497.9lumens m -2 [12].Day and night temperatures were held constant at 21˚C and 18˚C, respectively.At the beginning of the experiment, the day length was 11 hours and 35 minutes and gradually increased to 14 hours and 17 minutes by the end of the experiment [13].
Soil was sourced from the fallow phase of the 15-year long winter wheat -fallow rotation located at the James C. Hageman Sustainable Agriculture Research and Extension Center (SAREC) in Lingle, WY (42.14˚N, 104.35˚W, and 1272 meters above sea level).The area has a semi-arid climate with 125 frost-free days.Average annual precipitation ranges between 300 to 400 mm and average high and low temperatures are 15.2˚C and 0˚C, respectively (35-year averages) [11].Soils are loamy, mixed, active, mesic Ustic Torriorthents with <1% soil organic matter (SOM) and a maximum 3% slope [11,14].The top 10 cm of surface soil was collected with a hand trowel.Thirty individual sub-samples were placed in a bucket, homogenized by hand, and coarse rock fragments and visible roots removed.Soil was transported to the lab, air dried and sieved through a 2-mm sieve [15].

Experiment set up
Sixty plastic potting containers (volume of 2048 ml) were filled with approximately 1500 ml of soil.Twenty pots were amended with 72.6 g (dry bases) of composted cattle manure (compost).This amount is equals to a rate of 45 dry tons ha -1 and is based on recommendations from earlier studies that showed 11% release of N from compost in year one [11].Compost contained 1.24% N, 8.57% C, 0.85% available P, 0.89% iron (Fe), and had a C-to-N ratio of 7 (Table 2).Compost addition contributed 558 kg ha -1 total N and 382.5 kg ha -1 P. Compost natural abundance of N (δ 15 N) isotope was 10.4‰.Twenty pots were amended with inorganic fertilizer (Inorganic Fertilizer) at rates comparable to what local producers use and was comprised of mono-ammonium phosphate (NH 4 H 2 PO 4 ) or MAP (11-52-0), at a rate of .09Mg ha -1 and ammonium sulfate [NH 4 (SO 4 ) 2 ] (21-0-0) at a rate of 1.2 Mg ha -1 .The composition of the inorganic fertilizers are as follows: NH 4 H 2 PO 4 contains on average, 11% Nitrogen (N), 52% Phosphorus (P), and 0.6% Fluoride (F) and has δ 15 N of 1.7‰ [16]; while [NH 4 (SO 4 ) 2 ] contains 21% N and 24% Sulfur (S) and has δ 15 N of -0.9‰ [16].In all, the Inorganic Fertilizer treatment enriched soil with 351 kg ha -1 N, 207 kg ha -1 P, 288 kg ha -1 S, and 5.4 kg ha -1 F. The inorganic fertilizer treatment provided a comparison to compost to assess realized benefits of compost added at rates local producers apply.Inorganic fertilizer also acted as a proxy for readily available plant nutrients that compost provides.In addition to the compost treatment and inorganic fertilizer treatment, there was a control treatment with no soil additives (Control).
Cover crop mixes were formulated to perform and deliver specific management goals most suitable for the local weather and soil conditions.Five CC treatments were composed of legumes, broadleaves, and grasses(Table 3).The following CC treatments were planted at a low seeding rate as recommended by the cover crop vendor (Table 4) [16]: (1) phacelia monoculture (Phacelia tanacetifolia Benth.L.) (PH); (2) soil building mix (SB), nine species total (four legumes, one brassica, two broadleaf and two grasses); (3) nitrogen fixing mix (NF), ten species total (six legumes, one brassica, two broadleaf and one grass); (4) mycorrhizal mix (MM), 14 species total (five legumes, four broadleaf and five grasses) and ( 5), no cover crop control (CON).
Phacelia is a native annual forb from the Hydrophyllaceae family [17].It is fast germinating large broadleaf biomass producing form effective in competitive N uptake [18].Purple flowers attract beneficial insects such as hover flies (Syrphidae), that prey on aphids (Aphidoidea) [19] and is very pollinator friendly [17].
Prior to planting, pots were watered uniformly with 105 ml of purified water to adjust soil water content to 7% of water filled pore space (WFPS), which represents the normal soil water content for the field growing season [11].Pots were transferred to a greenhouse, arranged in a completely randomized design with four replications on a north-south longitudinal bench (Fig 1).Pots were rearranged weekly and soil moisture was adjusted daily to maintain constant 7% WFPS.This was done by comparing individual pot weights to a reference vegetation-free pot with soil kept at 7% WFPS (Fig 1).Based on daily soil water evaporation loss from the reference pot, all experimental pots were supplied with equal amounts water.

Plant and soil analyses
The experiment was terminated after nine weeks.Aboveground plant biomass was clipped and separated to CC and weedy species.All individual plants were identified and counted.Wet weights were recorded using Ohaus Scout Pro SP402 portable digital scale (Parsippany, New Jersey, USA).Aboveground biomass, separated by cover crops and weedy species, were stored in paper bags, oven-dried at 65˚C for 48 hours, and dry weights recorded.Soil and roots from each pot were emptied to a 2-mm sieve to separate roots and coarse fragments.Soil was homogenized and placed in a plastic zipper bag.All samples were kept in a cooler until laboratory analyses performed within 36 hours of collection.Soil pH and EC were tested using an Oakton 2700 series benchtop meter at a 1:2 soil-to-water ratio.Inorganic N was determined by adding 10 g of fresh soil to 25 ml of two molar potassium chloride (2 M KCl), placed on a shaker for 30 minutes, and filtered through ash-free filter paper (Q5 Fischer Scientific, USA).The extract was then analyzed on a spectrophotometer microplate reader (UV-VIS Biotek Instruments, Highland park, USA) for ammonium (NH 4 ) using sodium salicylate (Reagent A) and 2% bleach mixed with 1.5 M sodium hydroxide (Reagent B) [20].Nitrate was analyzed using the same procedure with vanadium chloride [21] as the sole reagent.Both parameters were then combined for a single estimate of inorganic N.
Potentially mineralizable nitrogen (PMN) was assessed by incubating five grams of fresh soil in 12.5 mL of DI water for 14 days at 25⁰C to create an anaerobic environment [22].Oxygen present in the headspace of the centrifuge tube was flushed with dinitrogen (N 2 ) gas prior to sealing the tube.After two weeks, 12.5 ml of 4 M KCl was added, shaken for 30 minutes, stored at 4⁰C overnight, and filtered through ash-free filter paper (Q5 Fischer Scientific, USA).The resulting extract was then analyzed using a spectrophotometer microplate reader (UV-VIS Biotek Instruments, Highland Park, USA), with PMN calculated as the difference between pre-and post-incubation NH 4 concentrations.
Dissolved organic carbon (DOC) concentrations were analyzed using the Newcomb-Carrillo method [23], with a 1:2.5 soil: K 2 SO 4 (0.5 M) extracts that were shaken for 30 minutes, stored at 4⁰C overnight, and filtered through ashless filter paper (Q5 Fisher Scientific, USA).Samples were analyzed on a total organic carbon (TOC) analyzer (Shimadzu TOC-VCPH with TNM-1, Japan).To calculate soil inorganic carbon (IC), a bicarbonate extraction was completed using 0.5g of air-dried, finely ground soil placed in glass medicine bottles.Two mL of 6M FeCl 2 was pipetted into small glass cylinders, then dropped into larger glass medicine bottles, which were sealed with a rubber septum and tightly crimped.The bottles were rigorously shaken, then allowed to rest for six hours before testing the bottles using a pressure calcimeter (Serta, 280E, USA).Soil IC was then estimated with the Sherrod method [24].Soil organic carbon was calculated as the difference of soil IC and total C.

Nitrogen isotopes
Soil, compost, and plant biomass material were finely ground using a Wiley Mill (Wiley Laboratory Mill, Model 4, Arthur H. Thomas Co., Philadelphia, USA) to pass through a 1-mm screen.Soil and compost samples (25-26 mg) and plant material (2.5-3 mg) were wrapped in tin capsules and analyzed for total carbon (TC), total nitrogen (TN), and stable nitrogen isotope ratio ( 15 N/ 14 N) on an isotope mass spectrometer (Costech 4010 Elemental Analyzer Thermo Delta Plus XP IRMS).Reproducibility as determined through replicate measurements was better than 0.001‰ for δ 15 N [25].Measurements were normalized based on the measured values of standards material (Glutamic acid, δ 15 N Air : 5.8‰ ± .15‰),relative to Vienna Pee Dee Belemnite (VPDB) and air, respectively, where: The δ 15 N (‰) were used to determine biological N fixation (BNF).As discussed above, atmospheric N fixation by bacteria living on leguminous roots provides a traceable N source [8].An average of the weedy control treatment δ 15 N (‰) was used as a proxy for BNF in treatments absent of legumes.This average δ 15 N (‰) was subtracted from each individual CC treatment δ 15 N (‰) to establish the δ 15 N (‰) from BNF, which was then divided by ten to convert per mil to percent.This percent was then multiplied by the treatment biomass to obtain biomass from BNF.

Statistical analyses
The experiment was set up in a completely randomized design with four replications.Soil moisture, soil organic C, plant biomass, total percent C in plant biomass, total percent N in plant biomass, biological nitrogen fixation, C:N ratio of plant biomass, δ 15 N (‰) in plant biomass were analyzed with R version 3.6.2[26].Cover crop treatment and soil additive treatment (compost and inorganic fertilizer) were fixed effects.Data were tested for normality using the Shapiro-Wilk test.Soil moisture, plant biomass, total percent Carbon, total percent Nitrogen, and biological nitrogen fixation were square root transformed to meet assumptions of normality and means back transformed for reporting.Carbon to Nitrogen ratio, Nitrogen isotope and soil organic carbon were normal and analyzed without transformations.Soil gravimetric and chemical characteristics, and stable isotope concentrations were assessed using two-way Analysis of Variance (ANOVA) with significance at a minimum of P � 0.05.No interactions between CC treatment and soil additive treatment were found for this data set.Means separations were performed using Tukey HSD at a minimum of P � 0.05.Regression analyses was used to compare the relationship of weed biomass to cover crop biomass for cover crop treatments [27].

Cover crops
Cover crop emergence was three to four times greater in NF, MM and SB than in PH (Table 5).Berseem clover and Persian clover established successfully while chickpea, field pea, spring lentil, chickling vetch and mung bean failed to germinate.Four out of five species in the NF treatment that failed to germinate were legumes and the fifth one was sunflower.Common vetch, crimson clover and berseem clover were the most frequently observed legumes that successfully germinated across all mixes containing legumes.As such, common vetch, flax, and oats dominated SB; oats and crimson clover dominated NF and berseem clover, phacelia, and oats dominated MM (Table 5).
All CC treatments and CON produced comparable aboveground biomass (Fig 2).Out of CC treatments, MM, NF, and SB had the greatest CC biomass compared with PH.The highest BNF was observed in control and compost amended soils.Soil in PH treatment had negligible BNF (Fig 3).

Carbon and nitrogen
Cover crops in SB and NF mixes had the highest plant tissue total C while CC in PH and CON had the lowest (Table 7).There were no observable differences in total N in the CC tissue.The resulting highest C:N ratio was in cover crops in SB mix (Table 7).In contrast, weeds had comparable tissue total C across all CC treatments.Tissue total N was comparable across all CC treatments than CON, which turned significantly higher than tissue total N in cover crops in MM mix (Table 7).Weed tissue C:N ratio was also the highest in MM and the lowest in Soil organic C ranged between 1.10% and 1.75% (Fig 5).In native soil, without a soil additive, SB had the highest SOC but was only significantly different than CON (Fig 5A).In the Compost treatment, the highest SOC concentrations were in SB or MM and were significantly different than NF.Percent SOC was lower when Inorganic fertilizer was applied but among CC treatments, no significant differences were found.

Discussion
The CC mixes used in this study were formulated to perform and deliver specific management goals.Some of the CC mixes included diverse species within the same plant family but not all those species were present at termination.Soil building mix had the fewest species among CC mixes and the lowest planting density but outperformed the other mixes among soil and plant parameters.When designing a CC mix, selection of species from the Poaceae, Fabaceae, and Brassicaceae families along with an additional broadleaf is more beneficial than increasing species diversity within a plant family.
The presence of CC reduced weedy species biomass in all CC treatments.Soil building mix and NF were the most effective.This is important in CC performance analysis because as cover crop biomass in SB and NF increases, weed biomass is reduced in an explainable manner.Significant weed biomass reduction through CC incorporation was also observed in other studies [29][30][31].Cover crop plants appear to compete well for light and N and suppress weed growth.Consequently, CC can be effective in exhausting the weedy species soil seed bank.Weedy species had the highest plant tissue N concentration when no CC were present.Previous research found that spring CC provided competition to weedy species by scavenging for N and had28% more biomass tissue N than fallow weeds [32].In this study, MM outcompeted the weedy species for N.This was observed through the elevated C:N ratio and low nitrogen concentration found in the weedy plant tissue.The CC species in the MM treatment were outcompeting the weedy species for soil N and therefore the weeds had less N in the plant tissue, resulting in a higher C:N ratio.
Soil building mix experienced the highest BNF in native soil followed by native soil amended with compost.Legumes present in SB, such as common vetch, were fixing N at a higher rate than other CC.The compost application benefitted MM and increased legume BNF in this mix.Biological N fixation was hindered by the inorganic fertilizer amendment.When inorganic fertilizer was applied in conjunction with cover crops, legume cover crops Table 6.Regression analysis of weed biomass (g m -2 ) by cover crop biomass (g m -2 ) for nitrogen fixer mix (NF) and soil building mix (SB).R 2 followed by one star indicates significance at p = .01level.utilized N from the fertilizer rather than biologically fixing atmospheric N. When N is readily available from inorganic fertilizer, it is not necessary for CC to establish the symbiotic relationship with rhizobia bacteria.These findings are consistent with other studies where the application of inorganic fertilizer reduced the presence of N fixing bacteria [33].This study measured and compared δ 15 N in native soil, soil amended with compost, soil amended with inorganic fertilizer, compost, CC plant tissue and weedy species plant tissue in order to better understand N competition between CC and weedy species.In the control treatment, BNF of legumes present in CC mixes resulted in CC δ 15 N that fell below the native soil δ 15 N.As rhizobia root bacteria begin to fix atmospheric nitrogen and then transfer that nitrogen to the legume plant tissue, the CC δ 15 N declines towards the air δ 15 N of 0 ‰.The weedy species δ 15 N signature was comparable to the native soil δ 15 N which indicated that weedy species were utilizing soil N.This is important to consider when designing a CC mixture for the purpose of N competition.Cover crop mixes dominated by legume species will not be as competitive with weedy species regarding N accumulation since leguminous CC will fix their own N and weedy species will have access to soil N. Incorporating a Poaceae specie to a CC mixture adds a specie that scavenges for N and competes with weedy species for soil N [32].

Treatment
Composted cattle manure used in this study had a δ 15 N of 10.4‰.This is a little lower enrichment of 15 N than reported elsewhere (17.4 ± 1.2‰) [34].This could be due to the cattle manure used in this study not composting as long as in other studies.The longer a manure composts, the more enriched in 15 N the manure becomes due to 14 N either volatilizing or being utilized by microbes [35,36].Microbes utilize the 14 N present in the compost as a nitrogen source since it is the lighter isotope which leads to an enrichment of 15 N in the composted cattle manure.Composted cattle manure is left with an enriched δ 15 N signature when compared to the soil δ 15 N signature.This enrichment provides a good tracer to see if weedy species and cover crops are utilizing the compost in equal measures.
When native soil was amended with compost that had a δ 15 N of 10.4 ‰, N uptake from the compost was evidenced in the higher δ 15 N of both weedy species and CC.Since both weedy species and CC display the higher δ 15 N, N competition was observed.All weedy species' treatments δ 15 N increased above the soil δ 15 Nline and were closer to the compost δ 15 N.The CC δ 15 N also increased in most treatments but not all.This is an indication of atmospheric fixation still taking place among legume species in the CC mix.While some CC species were competing for the N found in the compost, legumes in the same CC mix were fixing their own N.In future studies, CC species should be separated into legume and non-legume species prior to isotopic analysis.This would allow for N competition comparisons of non-legume species in a CC mix to weedy species.
The inorganic fertilizer used in this study was manufactured through the Haber Bosch process where atmospheric N is captured and utilized in the manufacturing of inorganic fertilizer.Because of this, the inorganic fertilizer δ 15 N signal is around 1‰, very similar to the signature of atmospheric N (δ 15 N of 0‰) [16].This also gave a credible tracer to observe how CC and weedy species utilized inorganic N. In the Inorganic Fertilizer treatment, the weedy species δ 15 N fell below the soil δ 15 N.The CC species δ 15 N also uniformly decreased signifying inorganic fertilizer N uptake and therefore, competition with weedy species.However, it is hard to tell how much is attributed to inorganic fertilizer and how much is attributed to N fixation since fixing and non-fixing CC species were present in the mixes and not analyzed separately at the time of termination.
In the control treatment, SOC increased when CC were present.Soil building mix had higher SOC in both the control and compost treatments, while MM had higher soil organic C in the compost treatment.This could be since both SB and MM had a higher grass to broadleaf ratio than NF and Phacelia.The fibrous grass roots could provide an increase in rhizodeposits through root exudates which would account for the increase in SOC.In the Inorganic Fertilizer treatment, SOC did not increase across CC treatments.This is consistent with other findings where inclusion of CC increased the SOC stock in cropland soils [37].In a meta-analysis, Poeplau & Don (2015) found that by incorporating CC, SOC increased at a rate of 0.32 Mg ha - 1 yr -1 to reach a maximum load of 16.7 Mg ha -1 [37].When CC were planted as an intercrop in an almond orchard, SOC increased at a rate of 0.5 Mg ha -1 yr -1 [38].This is beneficial for producers regarding soil improvement using CC.This is also beneficial to climatologist as a recommendation for climate mitigation through C capture strategies.

CC BNF ¼ ððAverage d 15 N
Weedy Control À d 15 N CC TrtÞ=10Þ * BM CC Trt Where: BM = Cover Crop Biomass Weeds BNF ¼ ððAverage d 15 N Weedy Control À d 15 N Weeds in CC TrtÞ=10Þ * BM Weeds in CC Trt Where: BM = Weed Biomass in CC Trt CON and PH.Cover crops planted in amended soils showed differential rates of BNF.In general, BNF ranged between 2 to 40 g m -2 in the control, 10 to 50 g m -2 in the Compost treatment and 1 to 15 g m -2 in the Inorganic Fertilizer treatment (Fig 3C).For CC, the highest BNF values were in MM in compost (Fig 3B) followed by SB in control (Fig 3A).Phacelia had the lowest overall values of BNF across all treatments ranging from 0 to 9 g/m 2 .The lowest overall values were observed in IF treatment (Fig 3C).Soil δ 15 N were 4.91 ‰ in the control treatment, 5.93 ‰ in the Compost treatment, and 5.09 ‰ in the Inorganic Fertilizer treatment (Fig 4).These soil δ 15 N (‰) act as a baseline to which the weedy species δ 15 N (‰) and CC plant tissue δ 15 N (‰) can be compared.In the control treatment, weedy species δ 15 N (‰), and PH δ 15 N (‰) hovered around the soil δ 15 N line (Fig 4A), whereas NF mix δ 15 N (‰), SB mix δ 15 N (‰) and MM mix δ 15 N (‰) and the PH weedy species δ 15 N (‰) were lower than soil δ 15 N (‰) (Fig 4A).In the Compost treatment, the δ 15 N of all observations increased (Fig 4B).Weedy species δ 15 N observations in NF, SB, and no cover crop were not different than compost δ 15 N (10.4 ‰).All CC δ 15 N (‰) were lower than compost δ 15 N (‰) and NF mix δ 15 N (‰) was lower than the soil δ 15 N (Fig 4B).In the Inorganic Fertilizer treatment, all CC treatments δ 15 N (‰) were not different than the soil δ 15 N (5.09 ‰), except NF mix (Fig 4C).All weedy treatments δ 15 N (‰) were lower than the soil δ 15 N (‰) except for weedy species δ 15 N (‰) in the no CC treatment (Fig 4C).

Table 7 . Plant total carbon, total nitrogen, carbon to nitrogen ratio (C:N) for cover crops and weeds in cover crop treatments (Control-no cover crop (CON), Pha- celia (PH), Soil building mix (SN), Nitrogen fixer mix (NF), and Mycorrhizal mix (MM)).
Means and standard errors followed by different lower-case letters are different among treatments at p � 0.05.